HLA-B27 (B27) is responsible for a large proportion of the genetic susceptibility to spondyloarthritis, particularly ankylosing spondylitis. Despite improvements in the treatment of these diseases, therapy remains inadequate and requires continual use of medications that carry definite risks. It is expected that a better understanding of pathogenic mechanisms, and specifically the role of B27, will lead to improved interventions and possibly a means of prevention. Studies funded by this grant have shown that B27 is a protein that misfolds. Using an animal model of spondyloarthritis, where HLA-B27 is expressed in rats (B27-Tg rats), we have shown that B27 misfolding causes stress in the endoplasmic reticulum (ER) of the cell, which in turn activates what is known as the unfolded protein response (UPR). Preliminary findings indicate that the B27-induced UPR causes macrophages to strongly overexpress a group of cytokines when stimulated by Toll-like receptor (TLR) ligands. We refer to this a UPR-TLR synergy. TLRs are pattern recognition receptors that allow cells to sense and respond to microbial products, and they play an important role in linking innate and adaptive immune responses. The cytokines overproduced by cells undergoing an B27-induced UPR may promote a chronic immune deviation resulting in inflammation, linking protein misfolding to an inflammatory disease. The studies proposed in Aim 1 will determine the effects of B27-induced UPR activation on the production of cytokines in immunoregulatory cells including macrophages and dendritic cells, and establish the UPR component of UPR-TLR synergy. In Aim 2 we will test whether UPR-TLR synergy causes activation of CD4+ T cells, and establish whether this occurs in B27-Tg rats during the development of inflammation. Studies in Aim 3 use a transgenic approach to modulate the HLA-B27-induced UPR to determine its role in inflammation. Downstream cytokines overproduced as a result of UPR-TLR synergy will also be targeted using a knockdown approach to determine their role in disease. These studies will significantly advance our understanding of HLA-B27 misfolding and the UPR in an animal model of spondyloarthritis. This work has the potential to drive translational studies in humans, and lead to the development of novel therapies targeting this pathological response.